naturally, one of the big questions is -- what causes it? what makes some children autistic? for a long time, the best understanding has been it's a function somehow, of genetics. but tonight, new research suggests that what researchers call "environmental factors" may play at least as big a role. >> i think this is a really exciting first step for us, you know, to really start to delve into the environmental risk factor for autism and i think the difference is that once identified you can actually do something about it so we can actually, potentially, start thinking about prevention down the road. >> the next step, obviously, in this, is to determine which environmental factors may be at play. the best guess right now, according to this, is that they include things that happen early on in life, such as the age of a child's parents when the child is conceived, low birth weight. whether a child is part of a multiple birth or, perhaps, any infections a mother may have during her pregnancy. >>> up next as "nightly news" continued on a tuesday night, if you think you know the ris